The article presents an overview of the compressed air energy storage gas turbines (CAES GT). The CAES GT works at low turbine inlet temperatures and is capable of fast start and loading 10 minutes to full load. It is characterized by high ramp rates up or down with minimum load as low as 10%, and flat heat rate for most of the load range. With exceptional low fuel input efficiency of 85%, the kW/hr equivalent Btu input to the compression cycle must be added, resulting in an overall efficiency of close to 55%, impressive versus a 300 MW GTCC plant that is required to load follow when integrating renewable energy source. The metamorphosis from that of a “peak shaving” unit to that of a grid support and renewable energy enabler is now complete. Advanced Reheat Gas Turbines such as the GT24/26 with higher operating temperatures offer further potential development for CAES.
Compressed Air Energy Storage (CAES) can consist of high pressure 1,000 psia stored air as an energy medium, and is most efficient when heated before expansion for power recovery. This requires a reheat power expansion turbine, only a concept 40 years ago. Today, reheat gas turbine technology is still valid with improvements for very high pressure 2,800 psia stored air for high density power recovery for 400 kW/ 1.0 pps mass flow, providing a 25% benefit for large CAES power plant.
The First Unit
This was a bold step for the Brown Boveri (BBC) engineers and the Noordwestdeutche Kraftwerke (NWK) AG of Hamburg, W Germany.
“The concept of using gas turbines for peak shaving by compressing air during off-peak hours and releasing it through a turbogenerator at peak load periods is not new. What is new is the acceptance of this design by at least one major user, Noordwestdeutsche Kraftwerke AG, of Hamburg W. Germany. They’ve ordered a 290 MW peaking plant from Brown Boveri & Cie, Mannheim, W. Germany, for installation at Huntorf in the Bremen/ Oldenburg area. Unit should see first commercial service in 1977.” (Late News from Gas Turbine lnternational September-October 1975 Issue.)
What was proposed was to fill two solution-mined salt caverns of five million cubic feet with high pressure compressed air by an axial flow inter-cooled compressor boosted up to 1,000 psia then after cooled before discharging into the caverns. When there was need for power, the stored air was released at 650 psia to a high pressure combustor, expanded and reheated for expansion through a low pressure (LP) turbine. In all, this required a high pressure turbine development of a very high pressure ratio 45:1, which the BBC engineers were confident of achieving.
The unit went into commercial operation in 1978 and, in the first 30 years of reliable service, more than 465 GWH of electricity was produced, including more than 8300 starts.
To deliver 290 MW of power in nine minutes and a rapid start of six minutes to full load, the engineers eliminated the exhaust recuperator to reduce the cost of the overall project which also included the solution mining of the two air storage salt caverns. During the planning stages in early 70's fuel prices were also very low, so rapid power delivery took precedence over efficiency.
Since the planning stages, two world oil crises increased prices substantially and consideration was given to adding a recuperator which would save about 1/3 of the fuel consumption and directly impact energy prices.
CAES in the USA
BBC was confident of the CAES market being driven in the USA and developed a 60 Hz unit of 220 MW as well as a smaller 50/110 MW unit that would both serve the 50 and 60Hz requirements for smaller units. These designs all included the reheat single shaft turbine with exhaust recuperation to substantially improve the heat rate to 4000 Btu/kW/hr or lower.
Within three years of successful Huntorf operation, BBC designed and built the 220 MW CAES plant turbo-machinery for the Soyland Power Cooperative Inc. of Decatur, Illinois. Sadly, a geological fault in the cavern was its Achilles heel.
The McIntosh CAES 110 MW plant in Alabama by Dresser Rand (D-R) and EPRI was based on this concept; basically half scale of Soyland Power Cooperative design. This concept has been successfully operating since 1991 at the Power South CAES power plant. The salt dome solution-mined cavern of 20 Mft3 can support 2600 MW/hrs of generation without recharging the cavern to full pressure of 1102 psia after being drawn down to 630 psia.
CAES GT Technology Today
Improvements: After 30 years of operation, the Huntorf unit rating upgraded 10.7% to 321 MW in 2007, by increasing the airflow 11.5% and inlet pressure by 2.5 %, the HP inlet temperature was reduced from 1022F to 914F.
The latest version of the McIntosh reheat GT design is now offered with 400 pps vs. 340 pps which the flow path through the HP and LP turbines could readily accommodate. The maximum rating of the two expanders can now achieve a range of 134 MW to 145 MW.
Decoupling the Compressor: The two current operating units are single shaft configurations where the generator/motor drives the compression train. Decoupling and using a separate motor provides a wider choice of compression optimization and overall flexibility of operations.
Cavern Storage Depth: Current projects are based on deep depleted gas fields, aquifers and domal salt structures 4000 ft or more in depth, requiring air storage pressures as high as 2800 psia. How to deal with very high storage pressure without having to design an entirely new machine?
Innovative VHP Reheat GT: D-R solved the problem elegantly by retaining the proven low temperature cycle, adding a third un-fired expansion stage as a pressure reducing turbine using pre-heated air from the recuperator, now with a rating capability of 160 MW.
CAES Reheat 3 Casing VHP, HP&LP Turbine: the illustration below shows the 3 coupled expanders of which the HP and LP are fired sequentially.
Wyoming Wind and Mega CAES Project
One of the current projects in development is the Path finder 2,100 MW wind power at the site of the best wind energy in the country: southeastern Wyoming. This is planned to expand to 3,000 MW. At Delta, UT, Burbank Water and Power and Path finder will jointly develop 1,200 MW of CAES utilizing a unique salt cavern formation of high strength and depth, suitable for several storage caverns. Each cavern will hold the equivalent volume of the Empire State building; delivering 300 MW for48 hrs (14,400,000 kW/hrs) before recharging is required. There are two CAES units per cavern, each capable of 160 MW gross with a 100% compression train of approx. 130 MW.
Wind Energy produced in Wyoming, or stored in the CAES system will be shipped from Utah on the 2,400 MW Southern Transmission System (STS) line to Los Angeles Department of Water & Power's balancing area for use by and sale to California utilities.
Additionally, there is development of the 538 mile, 500 kV HVDC Zephyr transmission line to connect Path finder wind power to Delta, Utah, at the location of the Intermountain Power Project (IPP), for a Commercial Operation Date of post 2020.
The Reheat Recuperated GT Cycle
Once again, the reheat concept demonstrates its merits. The CAES GT works at low turbine inlet temperatures, capable of fast start and loading 10 minutes to full load; high ramp rates up or down with minimum load as low as 10%, flat heat rate for most of the load range. With exceptional low fuel input efficiency of 85%, the kW/ hr equivalent Btu input to the compression cycle must be added, resulting in an overall efficiency of close to 55%, impressive versus a 300 MW GTCC plant that is required to load follow when integrating renewable energy sources. The metamorphosis from that of a “peak shaving” unit to that of a grid support and renewable energy enabler is now complete.
Advanced Reheat Gas Turbines such as the GT24/26 with higher operating temperatures offer further potential development for CAES.
What innovations will follow next?